Three field-proven methods, in order of reliability:
1. Boiling (most reliable). Bring water to a rolling boil for 1 full minute (3 minutes above 6,500 ft, where lower air pressure raises the temperature needed to kill pathogens). This kills bacteria, viruses, and protozoa. It does nothing for chemical contaminants.
2. Chemical disinfection. Iodine or chlorine dioxide tablets, or unscented household bleach (2 drops per liter of clear water, 4 drops if cloudy, wait 30 minutes). Effective against bacteria and viruses; less effective against giardia and cryptosporidium unless you extend contact time. Doesn't remove bad taste, sediment, or chemical contamination.
3. UV light (e.g. SteriPEN-style devices). Fast and effective against biological contaminants if the water is clear — UV can't penetrate cloudy or sediment-heavy water effectively, so pre-filter through cloth first. Requires batteries/charge, which is the real-world failure point.
None of these remove heavy metals, industrial chemicals, or salt. If you suspect chemical contamination, find a different source rather than trying to treat around it.